Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Conventional exhaust gas aftertreatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) system to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3) and a reducing catalyst. Generally in such conventional SCR aftertreatment systems, ammonia or a source of ammonia is injected into the aftertreatment system and mixed with the exhaust gas to partially reduce the NOx gases. The reduction byproducts of the exhaust gas are then fluidically communicated to the catalyst included in the SCR aftertreatment system to decompose substantially all of the NOx gases into relatively harmless byproducts which are expelled out of such conventional SCR aftertreatment systems.
In conventional selective catalytic reduction systems for aftertreatment of exhaust gas (e.g., diesel exhaust gas) urea is often used as a source of ammonia for reducing NOx gases included in the exhaust gas of IC engines (e.g., diesel exhaust gas). The urea or any other source of ammonia communicated into conventional aftertreatment systems can be deposited on sidewalls and/or components of the aftertreatment exhaust system.
The reasons for incomplete thermolysis of urea include short residence times due to space constraints of the aftertreatment system, low exhaust gas temperatures and impingement of urea or otherwise diesel exhaust fluid particles on solid surfaces. Particularly, the impingement of urea of low temperature walls results in incomplete decomposition of urea leading to formation of solid urea deposits on the surfaces of the aftertreatment system. Such deposits can further lower temperature of the walls on which such deposits leading to even more urea deposits. Formation, growth and depletion of deposits are influenced by operating parameters of the aftertreatment system including exhaust gas flow rate, exhaust gas temperature, ambient temperature, exhaust fluid spray characteristics and injection rate, and design parameters.
The urea deposits can adversely affect system performance by increasing the engine back pressure, Certain exhaust aftertreatment components, such as vanadium-based catalysts, can be particularly sensitive to such deposits. Such deposits can trigger diagnostic error codes, deactivate exhaust aftertreatment components, and/or increase backpressure experienced by the exhaust gas thereby, degrading the performance of conventional SCR aftertreatment systems.